The present invention relates to embedding particles in webs. More particularly, the present invention relates to a process for embedding particles in adhesive films.
Webs containing particles are well known. Typically these webs are films or tapes. Particle-containing films are generally made by dispersing particles into a film precursor before fashioning it into film form. The dispersion technique works well for solvent-based resins and for cross-linkable resins that have a low viscosity in their pre-crosslinked state. Issues with particle dispersion can generally be solved by selecting the processing parameters, such as film precursor viscosity and shear rates.
However, for hot-melt processed resins, particle dispersion can be difficult. If the particles are much smaller than the gaps in the processing equipment, there is little problem. For applications such as anisotropic conductive adhesives, it is not always desirable to use such small particles. Using small particles in these applications, bonding times can be long because of the time it takes for the adhesive to flow to the point where the film thickness equals the diameter of the small particles. It is advantageous to have particles that are closer in size to the adhesive film thickness. However, if the particle size approaches that of the various gaps in the processing equipment (including the compounding equipment and coating apparatus) there can be problems in mixing while maintaining particle integrity, and processing equipment damage can occur. In addition, it is sometimes desirable to have the particles protrude from the surface of the film, such as when making retroreflective films. When curable materials are used in a hot melt process, one must achieve a balance between providing a temperature high enough to yield a viscosity that enables mixing while keeping the temperature low enough to prevent premature curing.
There are known systems which place particles onto a film in a specific pattern as well as in a random pattern. Most involve a first step of separating the particles and a second step of transferring them to a web. Techniques include putting particles into pockets (Calhoun, et al. U.S. Pat. No. 5,087,494), passing particles through screens (Sakatsu, et al. U.S. Pat. No. 5,616,206), magnetic alignment with ferromagnetic particles (Jin, et al. U.S. Pat. No. 4,737,112; Basavanhally U.S. Pat. No. 5,221,417), magnetic alignment of any particle with ferromagnetic fluids (McArdle, et al. U.S. Pat. Nos. 5,851,644; 5,916,641), stretching a film with close-packed particles on it (Calhoun, et al. U.S. Pat. No. 5,240,761), and particle printing (Calhoun, et al. U.S. Pat. No. 5,300,340). Another method of transferring particles is taught in EP 0691660 by Goto et al. in which electroconductive particles are electrostatically charged to attract them to an adhering (xe2x80x9csilicone-based sticking materialxe2x80x9d) film through a screen in contact with the film. The screen (or mask) is electrically charged to attract the particles. In this case, the particles coat only those areas not screened off. The screen serves as a selective filter, allowing particles to pass through only in a pattern corresponding to the openings in the screen. The excess particles are brushed or vacuumed off of the screen. The gaps between the distributed electroconductive particles are filled with a photocurable or thermally curable resin to prevent inter-particle electrical connections. Upon curing the resin, the sticking material is stripped away with the mask from the particle filled resin to form an anisotropic electrically conductive resin. These techniques all require significant investment in equipment or various disposable or reusable parts that add cost to the resultant particle-embedded web. The present invention embodies a simpler implementation.
The particles in particle-embedded webs either control the level of adhesion of the film or provide additional utility. For example, if the particles are electrically conductive, a conductive adhesive film can be made. Conductive adhesive films can be used as layers in the assembly of electronic components, such as in attaching flex circuits to printed circuit boards and the like. Z-axis conductive adhesive films are useful in making multiple, discrete electrical interconnections in multi-layer constructions where lateral electrical isolation of the adjacent parts is required. In another example, the particles can be retroreflective, creating retroreflective films. If the particles have no inherent tackiness, the adhesion level of an adhesive web can be controlled by the level of particle loading. Also, the particles could be hollow spheres with encapsulated material, yielding a web with encapsulated material on or near the surface that becomes available upon use.
The invention is a dispenser for dispensing particles onto a surface. The dispenser includes a hopper for receiving particles. The hopper has an opening at its bottom. A screen having openings is located adjacent the opening of the hopper and a mover, located outside of the hopper, moves particles from the hopper, through the screen, and onto the surface.
The screen openings can be uniformly sized and spaced and sufficiently large to let the largest particles pass through while being dispensed yet sufficiently small to hold the particles back when the dispenser is not operating.
The mover can include a cylindrical brush covered with regularly spaced bristles. The size of the bristles can be smaller than the size of the openings of the screen, and as the bristles move over the surface of the screen, they protrude through the openings of the screen and draw particles through the screen to dispense them onto the surface. The brush is rotatable and the rotational speed is variable to vary the dispense rate of the particles. Also, the brush is movable between a first position away from the screen and a second position contacting the screen can be used.
The distance from the screen to the central longitudinal axis of the brush can be adjusted to adjust the force of the brush on the screen and the dispense rate of particles. Also, excess particles can be removed from the brush using a cleaning wire.
The invention is also a method of dispensing particles onto a surface. The method includes the steps of holding particles in a hopper. The hopper has a dispensing opening covered by a screen. The screen has openings that are uniformly sized and spaced and are sufficiently large to let the largest particles pass through while being dispensed yet sufficiently small to hold the particles back when the dispenser is not operating. The method also includes a step of rotating, outside of the hopper, a cylindrical brush covered with regularly spaced bristles that are adjacent the dispensing opening to protrude the bristles through openings of the screen and draw particles through the screen to dispense them onto the surface. The method also includes varying the dispense rate of the particles. This can be done by varying the rotation speed of the brush, adjusting the distance from the screen to the central longitudinal axis of the brush, or both.
The invention is also an apparatus for making a web with embedded particles. The apparatus can include a maker for making the web receptive to the particles; a dispenser for dispensing the particles onto the web; a disperser for dispersing the particles to minimize particle aggregation in the web and provide a substantially uniform dispersion of particles in both the longitudinal and transverse directions of the web; and an embedder for embedding the dispensed particles in the web.
The disperser can include a buffer for buffing the surface of the web after the particles are dispensed onto the web. The disperser can electrically charge the particles before they are dispensed onto the web such as by a voltage supply connected to the dispenser to charge the particles while they are in the dispenser. The disperser can also include grounding the web or charging the web with an opposite charge to that of the particles.
The apparatus can also include a static charge eliminator which eliminates static charges on the web. This can include a static bar located along the web path, ionizing the atmosphere around the web, or both.
The embedded particles on the web can be z axis conductive, retroreflective, peel adhesion controlling, abrasive, encapsulating or combinations of these.
The invention is also a method of making a web with embedded particles including making the web receptive to the particles; dispensing the particles onto the web; dispersing the particles to minimize particle clumping in the web; and embedding the dispensed particles in the web.
In this method the dispersing step can be buffing the surface of the web after the particles are dispensed onto the web; electrically charging the particles before they are dispensed onto the web; or both. The dispersing step can include grounding the web or charging the web with an opposite charge to that of the particles. The making the web receptive step can include heating. The method can also include eliminating static charges on the web using at least one of a static bar located along the web path; and ionizing the atmosphere around the web.